The application of hydrogen as a fuel or auxiliary fuel has extended beyond gas-phase conditions. This paper reports the development of a detailed chemical kinetic model for hydrogen oxidation in supercritical H2O/CO2 mixtures based on an existing gas-phase model. Revisions were made to thermodynamic properties and elementary reactions to account for the supercritical conditions. The effect of supercritical water (SCW) on selected elementary reactions was taken into consideration. The model for SCW oxidation of hydrogen was evaluated against experimental data in the literature. The model was able to reproduce satisfactorily the experimental results. The observed discrepancies were largely attributed to experimental artifacts, mainly deviation from an ideal plug flow. Key elementary reactions were identified by sensitivity analysis, and details of the oxidation mechanism were inferred by analyzing reaction paths. Finally, effects of the pressure and CO2 concentration on the model predictions were discussed.